The invention relates to improved coatings for paper for ink jet printing applications and in particular, to coatings which enhance ink drying and produce higher quality printed images.
Ink jet printing methods utilize printheads having orifices which eject ink droplets onto a print medium. For higher quality, higher resolution printing applications, the orifices of the printheads have been increased in number and their diameter significantly reduced in size. Accordingly, for full color printed images, inks are formulated to include dyes and/or pigments and various carriers and/or solvents which are resistant to drying or otherwise clogging the nozzle holes of the printhead. Such ink formulations, however often adversely affect the properties of the ink deposited on the print medium resulting in longer drying times and/or color mixing or bleeding of the images resulting in poorer quality images.
Various print media may be used for ink jet printing applications depending on the ink formulations. Such media include cellulosic webs, synthetic papers, polymeric films and the like. As advances in ink jet printing have occurred, specialty papers containing exotic coatings have been developed. Such specialty papers are often more expensive than uncoated papers and may contain coatings which are not compatible with the wide variety of ink formulations being used or developed for ink jet printing applications.
Despite the abundance of specialty webs for ink jet printing, cellulosic webs remain the most widely used print media. Cellulosic webs are made by conventional paper making techniques wherein a fibrous pulp is deposited on a wire screen and dried to form a web. Accordingly, the webs contain minute pores or voids between the cellulosic fibers for absorption of liquids therein. The porosity of the webs may be changed by use of specialty coatings such as clays and the like which may change the hydrophilic properties of the webs so that the webs absorb or repel aqueous and/or organic fluids which may be used as carrier fluids in ink formulations.
Ideally, it is desirable for only the carrier fluid of the ink formulation to penetrate into the web thereby depositing colorant on the outer surface of the web. Balancing the properties of the ink formulations so that the formulations are adaptable for use with a wide variety of print media is extremely difficult. It is even more difficult to provide ink formulations which may be used on uncoated or plain paper webs. Accordingly, a need exists for print medium which is adaptable to accept improved dye and/or pigment based ink formulations.
With regard to the above and other objects and advantages thereof, the invention provides a paper coating composition for ink jet printing applications. The composition includes from about 2 to about 10 parts by weight polyvalent metal salt, from about 0 to about 30 parts by weight deionized water, an effective amount of cationic amine polymer and surfactant and from about 35 to about 60 parts by weight of an ink penetrant promoter selected from the group consisting of 1,2-propanediol, dipropylene glycol and mixtures of 1,2-propanediol and dipropylene glycol.
In another aspect the invention provides a method for improving print resolution, the method including applying an aqueous penetrant promoter composition to a cellulosic web to provide an ink receptive coating and printing on the coated web. The promoter composition applied to the web contains from about 2 to about 10 parts by weight polyvalent metal salt, from about 0 to about 30 parts by weight deionized water, an effective amount of cationic amine polymer and surfactant and from about 35 to about 60 parts by weight of an ink penetrant promoter selected from the group consisting of 1,2-propanediol, dipropylene glycol and mixtures of 1,2-propanediol and dipropylene glycol.
In yet another aspect the invention provides a cellulosic web containing a penetrant promoter coating and ink. The coating is applied to the web prior to printing in an amount sufficient to promote ink drying and/or penetration of ink into the web so as to reduce ink bleeding. The promoter composition includes from about 2 to about 10 parts by weight polyvalent metal salt, from about 0 to about 30 parts by weight deionized water, an effective amount of cationic amine polymer and surfactant and from about 35 to about 60 parts by weight of an ink penetrant promoter selected from the group consisting of 1,2-propanediol, dipropylene glycol and mixtures of 1,2-propanediol and dipropylene glycol.
The term xe2x80x9cbleedingxe2x80x9d as used herein refers to the unintended mixing of colors or wicking of ink colorant into the web rather than remaining on the surface of the web. Webs which are highly absorbent of the colorant tend to produce low resolution images because each ink dot tends to spread due to the affinity of the web fibers for the colorants in the ink. If the ink colorant penetrates too far into the web, too much light may be scattered by the interstices of the upper portion of the web resulting in a lower contrast image. Accordingly, it is desirable that only the ink carrier be absorbed into the web while the colorant remains substantially on the surface of the web.
An important advantage of the invention is that the promoter composition is more environmentally friendly because of the inclusion of 1,2-propanediol or dipropylene glycol than previous compositions which contain diethylene glycol as the penetrant promoter. 1,2-Propanediol and di-propylene glycol are widely used in soaps and cosmetics and as such are relatively safer than diethylene glycol or other polyhydric alcohols. An unexpected benefit of the use of 1,2-propanediol and/or dipropylene glycol is that paper coating compositions containing such glycols exhibit increased ink drying rates which enable higher print speeds as compared to other ink penetrant formulations.